There might be a great difference between riding in groups on small winding roads, which I do every Tuesday if possible, and riding long distance on highways in groups. As to the former it keeps me on the toes constantly evaluating the turns, the other drivers' position, other traffic and a lot more in order to get the most out of the drive. But I always try to keep my focus on the road 2-300 m ahead to avoid following a fellow rider into the ditch.
As to the latter I have never done it but I know that after driving a while on the highway I loose the feeling of speed and get the feeling of existing in an envolope of my own. It is in no way an unpleasant feeling but potentially dangerous, so when ever it happens, I break the state by changing speed or taking a break.
Constantly reflecting on and evaluating your driving is a good precaution.
greetings from a grey and relatively warm north with mediocre driving conditions (there is a lot of dirt on the small roads in the rural districts and the roads are moist)